In E-UTRA (Evolved-UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access), single-carrier (SC)-FDMA (Frequency Division Multiplexing Access) is adopted as an uplink radio access scheme.
In this scheme, as shown in FIG. 1, users A, B, C, D and E in a cell perform transmission by using different time and frequency resources (the minimum unit is called a resource block), so that orthogonality among users is realized in the cell, and single-carrier transmission with low PAPR (Peak-to-Average Power Ratio) can be realized by assigning consecutive frequencies. As a result, power consumption of the mobile station can be reduced. In addition, compared to a case where multicarrier transmission is used, there is an advantage that coverage area can be enlarged. Also, since the scheduler of the base station determines time or frequency resources to be assigned based on propagation state of each user and QoS (data rate, required error rate, delay, etc.) of data that should be sent, so that each user is assigned time/frequency resources of good propagation state. As a result, there is an advantage that throughput increases.
In this scheme, an uplink control channel is used for transmission of CQI (Channel Quality Indicator) for realizing frequency scheduling and adaptive modulation/channel coding for the downlink data channel, and used for transmission of acknowledgement information for retransmitting downlink data and the like. The acknowledgement information (that may be referred to as ACK/NACK information hereinafter) is obtained by error detection such as Cyclic Redundancy Check (CRC), and includes positive acknowledgement (ACK) that is for a case when an error is not detected, and negative acknowledgement (NACK) that is for a case when an error is detected.
As mentioned above, since the single-carrier based radio access scheme is adopted in the uplink, it is necessary to multiplex CQI and ACK/NACK information using the same resources when a user transmits CQI and ACK/NACK information in the same subframe. In addition, corresponding to that, in the base station, it is necessary to change methods of reception processing between a case in which only CQI is transmitted and a case in which CQI and ACK/NACK information are multiplexed and transmitted.
By the way, even though CQI and ACK/NACK information should be fed back, when the mobile station transmits only CQI due to reception error in the downlink control channel, not only the base station cannot obtain the ACK/NACK information, but also the base station performs reception processing by determining that information in which CQI and ACK/NACK information are multiplexed has been received. Thus, proper processing cannot be performed. Especially, from the viewpoint of preventing transmission delay of the whole communication system and preventing deterioration of downlink throughput due to the delay, it is important to quickly grasp ACK or NACK. Therefore, in order to detect what information the mobile station has sent, DTX (discontinuous transmission) detection of ACK/NACK information is performed.
By performing DTX detection of the ACK/NACK information, there are advantages that the base station can properly receive CQI and ACK/NACK information, and can perform demultiplexing processing, the base station can properly select RV parameter for retransmission of downlink data, and can perform transmission power control for the downlink control channel, and the like.